1. Field of the Invention
The present invention relates to a reflective lighting optical system, and more particularly, to a reflective lighting optical system that can minimize a back focusing distance and a height of the system, and improve a lighting efficiency by disposing a wire grid type PBS (Polarized Beam Splitter) at an oblique angle with respect to a short side of an imager.
2. Description of the Related Art
Recently, it has become a general tendency that a display device is designed to be slim and lightweight while having a large-sized screen. Particularly, the realization of the display device having such a large-sized screen becomes a major task in the display field. To date, a projection TV is well known as a typical display device having the large-sized screen.
The projection TVs are classified into a cathode ray tube (CRT) projection TV and a liquid crystal display (LCD) projection TV. The LCD projection TVs can be further classified into a system using a transmissive LCD and a system using a reflective LCD (liquid crystal on silicon (LCoS)).
The system using the reflective LCD has an advantage in that a panel thereof can be inexpensively made as compared with the system using the transmittable LCD.
A conventional projection system and a lighting system will be described hereinafter in conjunction with the accompanying drawings.
FIGS. 1 to 4 show a variety of conventional 3-panel type reflective LCD lighting systems.
Referring first to FIG. 1, as one of lighting systems for a projection TV using a reflective LCD, a reflective lighting system of a 3-PBS system is designed such that light radiated from a lamp 1 is directed to a first dichroic mirror 2 via a condensing lens. Red and green light rays of the light directed to the first dichroic mirror 2 are reflected on the first dichroic mirror 2 while a blue light ray passes through the same.
The reflected red and green light rays are directed to a second dichroic mirror 3. The green light ray is reflected on the second dichroic mirror 3 while the red light ray passes through the same. The red, green and blue light rays are then incident to first, second and third PBS 4a, 4b and 4c, which are arranged before an R, G, B LCoS Panel.
The incident red, green and blue light rays are then reflected on the first, second and third PBS 4a, 4b and 4c, respectively, and directed to first, second and third LCoS panels 5a, 5b and 5c, respectively. The red, green and blue light rays respectively directed to the first, second and third PBS 4a, 4b and 4c are phase-shifted to pass through the first, second and third PBS 4a, 4b and 4c, respectively.
The red, green and blue light rays are synthesized by an X-prism and incident to a projection lens.
As described above, a light path of the reflective lighting system of the 3-PBS system is comprised of three stages; a first stage defined by the lamp 1 and the first dichroic mirror 2, a second stage defined by the second dichroic mirror 3, the second LCoS panel 5b and the second PBS 4b, and a third stage defined by the first and second LCoS panels 5a and 5c, the X-prism 6 and the first and third PBSs 4a and 4c. Such three stages cause a depth of the system to be increased.
Furthermore, the system needs a large number of parts such as two dichroic mirrors, one mirror, one relay lens for correcting a path difference between the red, green and blue light rays, three PBS, and one X-prism.
FIG. 2 shows another conventional 3-panel type reflective LCD lighting system of a color quad system using a color selector instead of the relay system.
The lighting system depicted in FIG. 2 is designed to utilize a color selector to eliminate a light path difference between red, green and blue light rays. That is, while light radiated from a lamp 7 passes through a first color selector 8a, a blue light ray is changed into secondary wave (S wave) while red and green light rays are outputted as primary wave (P wave).
The blue light ray changed into the secondary wave is reflected on a first PBS 9a to be directed to a second PBS 9b in front of a blue LCoS panel, and the red and green light rays of P wave transmit the first PBS 9a. 
The blue light ray is then reflected on the second PBS 9b, incident to a third LCoS panel 10c, and then is reflected on the third LCoS panel 10c so that the phase of the blue light ray is shifted. The phase-shifted blue light ray passes through the second PBS 9b, after which it is incident to a fourth PBS 9d via a fourth color selector 8d. 
The green light ray of the primary wave is changed again into the secondary wave while passing through a second color selector 8b and is then incident to a third PBS 9c. At this time, the red light ray of the primary wave is incident to the third PBS 9c without any change. Therefore, the green light ray is reflected on the third PBS 9c while the red light ray passes through the third PBS 9c, after which the red and green light rays are incident to first and second LCoS panels 10a and 10b, respectively.
The red and green light rays incident to the respective first and second LCoS panels 10a and 10b are reflected thereon to be phase-shifted. The green and red light rays that are phase-shifted are incident again to the third PBS 9c and synthesized, and made to have an identical polarizing state, after which they are incident to the fourth PBS 9d. 
The red, green and blue light rays directed to the fourth PBS 9d are synthesized by a P/S separation/synthesize property of the PBS. The synthesized light is incident to a projection lens.
As described above, since a light path of the 3-panel type reflective LCD lighting system of the color quad system is configured in a two stages structure without using the relay system, the structure thereof can be simplified. However, four color selectors and four PBSs are required, increasing the manufacturing costs.
Furthermore, in the course of the P/S separation/synthesize process by the PBS, there may be a photoelasticity problem causing the incident wave to have a different ray of polarized light when the incident wave is outputted.
In order to solve the above-described problems while enhancing lighting efficiency using illumination light having an optic angle, a lighting system using a wire grid type PBS depicted in FIG. 5 has been proposed.
FIG. 3 shows a conventional lighting system using a wire grid type PBS.
As shown in the drawing, a lighting system is configured such that light irradiated from a lamp 11 is directed to a first dichroic mirror 12a via a condensing lens. Red and green light rays of the light directed to the first dichroic mirror 12a pass through the first diebroic mirror 12a while a blue light ray is reflected on the same.
The red and green light rays passing through the first dichroic mirror 12a are changed into primary and secondary waves, respectively, while passing through a color selector 14 and are then directed to a second wire grid type PBS 13b. The red light ray changed into the primary wave passes through the second wire grid type PBS 13b while the green light ray is reflected thereon, after which the red and green light rays are incident to the first and second LCoS panels 15a and 15b, respectively, and are then phase-shifted by being reflected on the first and second LCoS panels 15a and 15b, respectively. The red and green light rays that are phase-shifted are incident to a projection lens via the second wire grid type PBS 13b and the second dichroic mirror 12b. 
In addition, the blue light ray reflected on the first dichroic mirror 12a is directed to a first wire grid type PBS 13b and is then reflected thereon to be directed to a third LCoS panel 15c. The blue light ray directed to the third LCoS panel 15c is phase-shifted by being reflected thereon and is then incident to the projection lens after being reflected on the second diachronic mirror 12b via the first wire grid type PBS 13a. 
FIG. 4 shows another conventional lighting system using a wire grid type PBS.
As shown in the drawing, a lighting system is designed such that light irradiated from a lamp 16 is directed to a first dichroic mirror 17 via a condensing lens. Red and green light rays of the light directed to the first dichroic mirror 17 are reflected thereon while a blue light ray passes through the same.
The blue light ray is incident to a third LCoS panel 21c via a second relay lens 18b, a reflective mirror, a third relay lens 18, and a third wire grid type PBS 20c. 
The blue light ray is then incident to an X-prism 22 via the third wire grid type PBS 20c after it is reflected on and phase-shifted by the third LCoS panel 21c. 
The reflected red and green light rays are directed to a second dichroic mirror 19 via a relay lens 18a. The green light ray is reflected on the second dichroic mirror 19 while the red light ray passes through the same.
The reflected green light ray is reflected on a second wire grid type BPS 20b and is then incident to a second LCoS panel 21b. The green light ray is phase-shifted by the second LCoS panel 21b to be directed to the X-prism via the second wire grid type PBS 20b. 
The red light ray passing through the second dichroic mirror 19 is reflected on a first wire grid type BPS 20a and is then incident to a first LCoS panel 21a. The green light ray is phase-shifted by the first LCoS panel 21a to be directed to the X-prism via the first wire grid type PBS 20a. 
The red, green and blue light rays are directed to a projection lens after being synthesized by the X-prism.
As shown in FIG. 5, the wire grid type PBS is composed of a glass plate on which a predetermined pattern is formed.
The predetermined pattern formed on the glass plate has a size of several tens nm.
When the lighting system is structured using such a wire grid type PBS, a variety of problems such as a photoelasticity problem, a cost problem and a lighting efficiency problem can be solved. However, an astigmatism problem is incurred.
That is, when the glass plate is inserted in the imaging lens system at an oblique angle, astigmatism that prevents light rays from focusing clearly at one point on the retina, resulting in blurred vision, is generated.
Particularly, the astigmatism becomes more severe when the light passes through the wire grid type PBS after being reflected on the LCoS panel.
Referring to FIG. 3, the green light ray reflected on the second LCoS panel 15b passes through the second wire grid type PBS 13b, and the blue light ray reflected on the third LCoS passes through the first wire grid type PBS 13a. 
Referring to FIG. 4, the light rays are reflected on the first, second and third LCoS panels 21a, 21b and 21b pass through the first, second and third wire grid type PBS 20a, 20b, 20c. 
As described above, when the light ray passes through the wire grid type PBS after being reflected on the LCoS panel, the astigmatism is incurred.
The astigmatism incurred when the light ray reflected on the LCoS panel passes through the wire grid type PBS will be described hereinafter with reference to FIGS. 6 to 8.
FIG. 6 shows a layout diagram of the projection lens when the light ray passes through the wire grid type PBS, while FIGS. 7 and 8 show a wave shape in the case of FIG. 6.
A case where a light rays passes through a wire grid type PBS inserted between a projection lens and an LCoS panel at an oblique angle will be described hereinafter.
Aberration incurred when the light ray passes through the wire grid type PBS inserted between the projection lens and the LCoS panel at the oblique angle as shown in FIG. 6 is as shown in FIGS. 7 and 8.
That is, as shown in the drawings, there is a problems that astigmatism is incurred when the light ray passes through the wire grid type PBS inserted between the projection lens and the LCoS panel at the oblique angle.
As described above, the conventional reflective lighting systems have a variety of problems.
That is, the reflective lighting system of the 3-PBS system as illustrated in FIG. 1 has problems that the depth of the system is increased due to the three light path portions and a number of components are required.
Although the reflective lighting system of the color quad system as illustrated in FIG. 2 has an advantage of a simplified structure due to the two light path portions, it has a problem that the manufacturing cost is increased due to the four color selectors and the four PBS.
Furthermore, in the course of the P/S separation/synthesize process by the PBS, there may be a photoelasticity problem causing the incident wave to have a different ray of polarized light when the incident wave is outputted.
Although the reflective lighting systems each using the wire grid type as illustrated in FIGS. 3 and 4 solve the problems such as the photoelasticity problem, the cost problem and the lighting efficiency problem, it has an astigmatism problem.
In order to reduce the astigmatism, a scheme for reducing a thickness of each of the wire grid type PBS or a scheme for disposing two wire grid type PBSs in a different direction has been proposed.
However, when the thickness of the PBS is reduced, the glass plate may be deformed. In addition, even when the PBSs are disposed in a different direction, the astigmatism is not compensated for, but a shape of a spot is formed in a circular-shape, increasing the size of the spot. Furthermore, since the PBSs have a different angle from each other, the lighting system cannot be structured on a single plane.